1. Field of the Invention
The present invention relates to power converters, and more particularly, relates to a switching controller of power converters.
2. Description of Related Art
FIG. 1 schematically shows a circuit of a boost power converter. An input voltage VIN is supplied to an inductor 20. A rectifier 30 is connected between the inductor 20 and an output of the boost power converter to provide an output voltage VO. The output voltage VO is a boosted voltage which is higher than the input voltage VIN. The output voltage VO is used to power a load 46, for example, a PWM switching circuitry. A power switch 10 coupled to a joint of the inductor 20 and the rectifier 30 performs energy switching to regulate the output voltage VO.
In conventional boost power converters, when the input voltage VIN is 90VDC, for example, the output voltage VO will be boosted up to around 380VDC irrespective of load conditions of the load 46. That is, the conventional boost power converter maintains its output voltage VO at 380 VDC as the load 46 decreases to a light-load condition. This lowers the conversion efficiency because a sufficient level of the output voltage VO on the light-load condition would be only 300 VDC, for example. Redundant power consumption is therefore wasted on the switching loss of the power switch 10 and the power loss of the rectifier 30.
In order to avoid redundant power consumption as mentioned above, FIG. 2 shows a conventional switching controller 100a to solve this problem. Referring to FIG. 1, the boost power converter comprises a switching controller 100. A feedback terminal FB of the switching controller 100 is coupled to the output of the boost power converter via a voltage divider 50. The voltage divider 50 comprises two resistors 51 and 52 connected in series between the output of the boost power converter and a ground reference. A capacitor 40 is connected to the voltage divider 50 in parallel. An output signal VFB received at the feedback terminal FB is obtained at a joint of resistors 51 and 52.
FIG. 2 schematically shows a circuit of the switching controller 100a of the boost power converter. The switching controller 100a comprises a switching-control circuit. The switching-control circuit comprises an oscillator 110, an inverter 150, a flip-flop 155, an AND gate 160, and a comparator 250. The oscillator 110 generates a pulse signal PLS and a ramp signal RMP. The flip-flop 155 receives a power source VCC, and clocked by the pulse signal PLS via the inverter 150 to enable a switching signal VSW at a terminal SW. The output signal VFB is supplied to a negative input of an error amplifier 200a via the feedback terminal FB. A first terminal of a first switch 120 and a first terminal of a second switch 130 are both connected to a positive terminal of the error amplifier 200a. A second terminal of the first switch 120 is supplied with a first reference voltage VH. A second terminal of the second switch 130 is supplied with a second reference voltage VL. The second reference voltage VL is lower than the first reference voltage VH. The first switch 120 is controlled by a power-saving signal SE via an inverter 140. The second switch 130 is controlled by the power-saving signal SE. The error amplifier 200a generates a feedback signal VCOM transporting to a compensation terminal COM which is connected to a capacitor 45. The comparator 250 compares the ramp signal RMP and the feedback signal VCOM. Once the ramp signal RMP is higher than the feedback signal VCOM, the flip-flop 155 is reset and the switching signal VSW is disabled.
The switching controller 100a further comprises a comparator 380 to compare the feedback signal VCOM and a light-load threshold VTH. On normal/heavy load conditions, the first switch 120 is turned on and the positive terminal of the error amplifier 200a is supplied with the first reference voltage VH. As the load condition of the load 46 decreases, the output signal VFB increases responsively, which decreases the feedback signal VCOM. Once the feedback signal VCOM is lower than the light-load threshold VTH, the comparator 380 generates a power-saving signal SE to turn on the second switch 130. The power-saving signal SE also turns off the first switch 120 via the inverter 140. Thus, the positive terminal of the error amplifier 200a is supplied with the second reference voltage VL instead. This further generates a lower feedback signal VCOM compared to the feedback signal VCOM when the positive terminal of the error amplifier 200a is supplied with a first reference voltage VH. The lowered feedback signal VCOM shortens an on-time of the switching signal VSW. Therefore, the output voltage VO decreases to, for example, 300 VDC to sufficiently power the load 46.
When the output signal VFB at the negative terminal of the error amplifier 200a decreases due to an increased load condition of the load 46, the feedback signal VCOM increases responsively. As the feedback signal VCOM is higher than the light-load threshold VTH, the power-saving signal SE is disabled and the positive terminal of the error amplifier 200a is supplied with the first reference voltage VH again.
This further increases the feedback signal VCOM. The on-time of the switching signal VSW is expanded and the output voltage VO is again boosted to, for example, 380 VDC to sufficiently power the load 46.
As mentioned above, the conventional art is able to solve the problem of redundant power consumption of boost power converters on the light-load condition. However, the determined level of the output voltage VO in the conventional art on the light-load condition is fixed because it is restricted by the reference voltages VL embedded in the switching controller 100a. This restricts the boost power converters to meet their practical power-saving requirements, for instance, to further reduce the determined level of the output voltage VO on the light-load condition from 300 VDC to 260 VDC. The conventional art fails to externally programming its determined output voltage level required whenever needed.
As a result, there is a need to provide a boost power converter not only capable of regulating the output voltage VO in response to different load conditions but capable of externally programming a determined output voltage level to meet practical power-saving requirements.